Conventional telecommunication cables other than those of the coaxial type comprise, within a common flexible envelope, a multiplicity of conductor arrays each consisting of either two conductors (referred to as a balanced pair) or four conductors (referred to as a quad). The conductors of a pair are helically intertwined; two such pairs may be twisted together to form a quad of the kind known as DM (Dieselhorst-Martin) type. Arrays in which four conductors are twisted about a common axis are referred to as star quads. A quad may also consist of two pairs of twisted conductors extending alongside each other.
A quad thus comprises two conductor pairs each forming a signaling circuit which is balanced with reference to ground. A third signaling circuit, known as a phantom circuit, consists of the conductors of one pair connected in parallel for transmission in one sense and the conductors of the other pair connected in parallel for transmission in the opposite sense.
The close juxtaposition of the conductors within an array establishes distributed capacitances therebetween which, if not precisely balanced, give rise to spurious signals resulting in interference phenomena (i.e. cross-talk) among the several circuits. If the resistance of the wire insulation is low, leakage currents cause increased attenuation of the transmitted signals along the line.
The interconductor capacitances within a cable, whose imbalance is primarily responsible for the cross-talk, is determined not only by the dielectric constant of the insulating material but also by the geometry of the conductor array. Thus, any departure from geometrical symmetry among these conductors leads to a capacitive imbalance. That symmetry, however, is difficult to maintain with wires which are coated with thermoplastic sheaths extruded therearound, such extruded conductors being generally utilized nowadays in lieu of earlier constructions wherein the insulation consisted of paper strips helically wrapped around the wire. Drawbacks of these earlier insulations included their slow rate of production, their fragility and the difficulty of splicing them. The extruded conductors, on the other hand, have the disadvantage of relatively low friction which allows them to slide within the cable envelope during subsequent handling and impairs their original symmetry; also, the dielectric constant of the thermoplastic insulation is relatively high, thus requiring an increased sheath thickness to minimize the line attenuation due to leakage currents.
Moreover, quads of plastic-coated wires are ineffectual in resisting the propagation of moisture in the event of a rupture of the cable envelope. Usually, the entry of water onto the cable does not manifest itself in a significant immediate loss of signal strength and therefore often remains undetected until the water has permeated a considerable length of the cable and has wrought extensive damage. Attempts to limit the propagation of moisture inside a cable by gas pressure or viscous materials, such as petroleum jelly, are costly and cumbersome.